X-ray standing wave investigations of Si dopant incorporation in GaN
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X-ray standing wave investigations of Si dopant incorporation in GaN M. Siebert1 , Th. Schmidt1 , J. I. Flege1 , J. Zegenhagen2 , T.-L. Lee2 , S. Figge1 , D. Hommel1 and J. Falta1 1 Institute of Solid State Physics, University of Bremen, Otto-Hahn-Allee, 28359 Bremen, Germany 2 ESRF, Polygone Scientifique Louis N´eel, 6 rue Jules Horowitz, 38000 Grenoble, France ABSTRACT The synchrotron radiation technique of x-ray standing waves (XSW), which allows to directly obtain structural and element-specific data, was successfully used for the investigation of the dopant site distribution in Si doped GaN films grown on (0001) sapphire substrates for the first time. The Si dopant concentration was chosen to 5 × 1018 cm−3 and 5 × 1019 cm−3 . The measurements were performed on 300 nm thin doped films deposited on high-quality thick undoped GaN films. With this sample structure, influences of the Si dopant induced decrease of the crystalline quality on the XSW signal are suppressed. The XSW data are compared to those obtained from thick homogeneously doped GaN films. All XSW measurements were performed in (002) backscattering geometry. Independent of the dopant concentration, the results indicate that Si atoms are solely incorporated on substitutional Ga sites. INTRODUCTION Silicon is the material most commonly used for n-type doping of GaN crystals. However, due to the high tensile strain which is induced by Si doping, the GaN crystal quality significantly decreases with increasing Si incorporation [1]. This is accompanied by the formation of a variety of defects, thus the lifetime and the efficiency of Si doped GaN based lasing devices are limited. In this paper we investigate the interdependence between the dopant incorporation site distribution as depending on the dopant concentration and the defect formation. We report on the possibility to apply x-ray standing waves (XSW) on Si doped GaN samples in order to investigate the dopant site distribution. EXPERIMENTAL METHOD XSW measurements rely on the fact that in the proximity of Bragg’s diffraction condition a standing wave field is established in the overlap region between the incident and the outgoing photon beam, exhibiting a periodicity given by the lattice spacing dhkl [2]. While scanning either the incident angle θ with respect to the diffraction planes or the incident photon energy E through Braggs’s diffraction condition for a reflection (hkl), the phase between the incoming and the ¯ k¯¯l) outgoing beam changes by −π. Therefore, the standing wave field pattern translates in (h direction by 12 dhkl . Hence, the intensity of an element-specific inelastic signal is a function of θ or E, respectively. The evaluation of this function according to the dynamical theory of diffraction [2, 3] yields the coherent fraction fc and coherent position Φc , which are the modulus
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and phase, respectively, of the (hkl) Fourier component of the distribution of atoms that contribute to the inelastic signal yield. While XSW measurements on nearly perfect
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